Chest
Volume 111, Issue 1, January 1997, Pages 188-193
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Clinical Investigations in Critical Care
Applied PEEP During Pressure Support Reduces the Inspiratory Threshold Load of Intrinsic PEEP

https://doi.org/10.1378/chest.111.1.188Get rights and content

Objectives

Mechanical ventilation in patients with obstructive airway disease (OAD) is associated with the development of dynamic hyperinflation and intrinsic positive end-expiratory pressure (PEEPi). One of the effects of this form of PEEPi is to act as an inspiratory threshold load that can produce ineffective breath triggering, dyspnea, and muscle fatigue. Recently it has been shown that applying PEEP in the ventilator circuit can reduce this imposed triggering load. We wished to investigate this further by studying patients with OAD being weaned with pressure support (PS) ventilation. Our first objective was to determine the prevalence and magnitude of this form of PEEPi in OAD patients who were clinically judged to be capable of triggering mechanical ventilatory breaths. Our second objective was to attempt to reduce the triggering load by applying circuit PEEP and then observe the response of patient-ventilator interactions during the patient-triggered, pressure-limited PS breath.

Design

Thirteen random patients with OAD who were receiving PS ventilation were studied by measuring airway pressures, airway gas flow, baseline esophageal pressure, esophageal pressure time products (PTP), and esophageal pressure changes before ventilator gas delivery began (APes taken to represent PEEPi). Measurements were made at baseline and after stepwise increases in circuit PEEP up to the PEEPi.

Results

We found measurable PEEPi in all patients (average ±SD of 9.54±4.3 cm H2O) and it was > 10 cm H2O in seven patients. As would be predicted, we observed progressive reductions in PEEPi as applied PEEP was given. We also observed that the component of patient effort (PTP) related to overcoming PEEPi also decreased, but the PTP related to tidal volume (VT) did not. The VT associated with the set PS thus did not change with application of PEEP, nor did the breathing frequency.

Conclusion

PEEPi is common in OAD patients receiving mechanical ventilatory support. The imposed triggering load from PEEPi can be offset to large extent by circuit PEEP approaching the baseline PEEPi. Although total patient effort substantially falls with applied PEEP, the patient effort that combine with PS to effect VT does not.

Section snippets

MATERIALS AND METHODS

Patients requiring mechanical ventilatory support for OAD over a 12-month period in our medical ICU were eligible for study. To be selected, patients had to be the following: (1) in clinically stable condition; (2) judged not ready for extubation; and (3) on a stand-alone PS ventilator weaning protocol. In our institution, the PS ventilator weaning protocol is instituted when patients recover their spontaneous ventilatory drive and the underlying acute disease has stabilized or is reversing.

RESULTS

Thirteen patients were studied with a mean (±SD) age of 58±3.8 years. Seven patients were men. Two patients had a diagnosis of asthma and 11 had a diagnosis of COPD. The baseline physiologic data are given in Table 1. As can be seen, these patients had mild baseline hypercarbia and high minute ventilation needs (mean delivered minute ventilation of 13.9 L/min). All patients had a measurable APes (PEEPi) with the mean being 9.4 cm H2O. Seven of the 13 patients (54%) had values greater than 10 cm

DISCUSSION

Alveolar pressure can remain above ventilator circuit pressure at end expiration in mechanically ventilated patients with obstructive airway disease through several mechanisms. These include narrowed collapsing airways (flow limiting segments) that can both retard or prevent expiratory flow,1, 2, 3, 4, 5 an inadequate expiratory time for the delivered tidal breath,3, 15, 20 and expiratory muscles remaining active up to the point of breath initiation.7, 21

Pes measurements are often taken as a

ACKNOWLEDGMENT

The authors are indebted to Janet Johns for her secretarial help.

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    Partially supported by a research grant from Bicore Inc, Irvine, Calif (now Allied Health Care Products, Riverside, Calif).

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